Here is some helpful information if you want to modify your stock truck
wiring harness your self. This junction block is part of the main engine
harness, that screws into the bottom of under-hood fuse block. If you
want to use the whole fuse block you can, but I like to keep it clean, and
wire a lot of these together. There are a lot of wires here. Each
injector has its own power wire all the way to this block. All 8 of them
can be wired together into a single fuse to clean things up. This is what
is done inside the fuse block. Be sure to look at the
pictures of the harness at bottom of this page.

Click Year for Pinouts for Truck Harnesses.
Some of the wires inside these links are made BOLD. This typically
means this wire is needed or very useful in a stand alone harness setup.
C2 information is in these links. Info is more complete for 1999, 2000.
Later years should be similar, use circuit #'s to compair. 1999-2002 are
VERY similar. Around 2001, the engines no longer had EGR, so that does
away with 5 wires. The differences in the 99-02's don't matter a bit when
making a harness stand alone. Very detailed info for what to remove, keep,
etc is just a few lines down, I leave all this info so you can see changes year
to year if you lik.

All the grounds needed will be in the engine harness. Should be a few
of them. Rear of engine behind intake, should be two grounds. G103 & G104,
behind accessory bracket is a ground. G102

The very first step I do when modifying these Vortec harnesses, is cut all
the wires on C100, C152, and cut all BUT the PINK from C2 Underhood. Next
step is remove the pins from the PCM connectors you don't need, and label those
you DO need. See pinout charts listed here for 99-02
or 03-07 I have color coded the PINS TO REMOVE
and the PINS TO
EXTERNAL CONNECTION (KEEP).

For 1999-2007!! here is a description of the wires from the PCM connectors thatwill
connect to an external connection, outside the wiring harness. This is
all you need for stand alone. (Includes Engine, Transmission) No
Emissions.

Pin 33 - Purple - TCC Brake
Switch - This wire tells the PCM when you hit the brakes. It needs 12v+ all
the time, and the brake switch should "open" this circuit when you hit the
brakes.

Pin 42 - empty - Fan 1
Control - This will be empty on 99-2002 harnesses. A pin can be added for
electric fan control. Programming will enable the fan control. PCM
supplies a GROUND to turn on a relay. If you have two fans, keep reading,
pin for fan 2 will be on the RED connector C2. You must wire your fans through a
relay.

Pin 9 - Dk Green/White -
Fuel Pump Relay Control - This wire the PCM supplies 12v+ to control a relay
for fuel pump. PCM will turn on the fuel pump for 2 seconds at initial key
on. When the engine cranks or starts, the fuel pump will then continue to
run, untill the engine stops, or you shut off the key.

Pin 10 - White - Engine
Speed Signal - Tach signal. Stock programming of the PCM will output a
4 cylinder tach signal. When I program the PCM, I can change this signal
to a 8 cylinder. This way you don't have to modify your tach.

Pin 33 - Dk Green - Fan 2
Control - some 99-02 harnesses will have a pin here, this was used for the
HVAC Recirculation Door control. The PCM would control the A/C
recirculation to help cool the a/c system if it got too hot. In an engine
swap, programming can be changed to let this control a second cooling fan.
Again, PCM supplies a GROUND to run on a relay. You must wire your fans through
a relay.

Pin 46 - MIL Control -
This is your ''check engine'' light control wire. PCM will supply a GROUND
on this wire when the light should be ON.

Pin 50 - Dk Green/White -
VSS - This is a signal generated by the PCM, based on information it
receives from the vehicle speed sensor located in the transmission. The
PCM takes the sensor signal, and calculates the tire size, gear ratio programmed
into it, and makes a 4000 pulse per mile signal. This is useful if you
want to use a cruise control box. Some speedometers will use this as an
input, others, like Autometer's, will hook directly to one wire of the speed
sensor in the trans.

After pins are removed, AND some labeled, I start working into the harness to remove un-needed wires
that are now removed from the PCM connectors. Some will pull right out, as
they went to C100, C152, or C2 Underhood. Pull all the loose wires that go
deeper into the harness back past where the original C100, C152 were located.
Tightly tape up the wire bundle at this location, as to not let there be too
little or too much slack on the wires going to the PCM. Be sure to use masking tape to
hold the basic shape of the engine harness. As you progress into the
harness, you will find some ground wires that trace back to splice packs.
Simply cut the wire up close to the splice pack. Keep 1 or 2 ground wires
for when you add the relays and diagnostic port, you will need a ground.
DO NOT LET THE HARNESS FALL APART OR GET TANGLED. As I get up to where
wires branch out, I will follow that branch out, remove any wires going into it,
then tape that branch back up. Only doing one branch at a time.

You can start to cut and label the pink wires from C2 Underhood. See
this link and look for the wires
highlighted in BLUE. You may find
it easier to use a digital volt-ohm meter set to continuity beep mode, in
finding where each pink wire goes. This is pretty standard across all
Vortec 4.8/5.3/6.0 harnesses: PCM - 2 pink; MAF - 1 pink; TRANS - 1; COILS - 2;
INJECTORS - 8; O2 SENSORS 1 EACH;